During the drilling of a wellbore, various fluids are typically used in the well for a variety of functions. The fluids may be circulated through a drill pipe and drill bit into the wellbore, and then may subsequently flow upward through wellbore to the surface. During this circulation, a drilling fluid may act to remove drill cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when circulation is interrupted, to control subsurface pressures, to maintain the integrity of the wellbore until the well section is cased and cemented, to isolate the fluids from the formation by providing sufficient hydrostatic pressure, to prevent the ingress of formation fluids into the wellbore, to cool and lubricate the drill string and bit, and/or to maximize penetration rate.
For a drilling fluid to perform these functions and allow drilling to continue, the drilling fluid has to stay in the borehole. Frequently, undesirable formation conditions are encountered in which substantial amounts or, in some cases, the drilling fluid may be lost to the formation. Drilling fluid can leave the borehole through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole.
Lost circulation is a recurring drilling problem, characterized by loss of drilling mud into downhole formations. However, other fluids, besides “drilling fluid” can potentially be lost, including completion, drill-in, production fluid, etc. Lost circulation can occur naturally in formations that are fractured, highly permeable, porous, cavernous, or vugular. These earth formations can include shale, sands, gravel, shell beds, reef deposits, limestone, dolomite, and chalk, among others.
Providing effective fluid loss control without damaging formation permeability in completion operations has been a prime requirement for an ideal fluid loss-control pill. Conventional fluid loss control pills include oil-soluble resins, calcium carbonate, and graded salt fluid loss additives, which have been used with varying degrees of fluid loss control. These pills achieve their fluid loss control from the presence of solvent-specific solids that rely on filter-cake build up on the face of the formation to inhibit flow into and through the formation. However, these additive materials can cause severe damage to near-wellbore areas after their application. This damage can reduce production levels if the formation permeability is not restored to its original level. Further, at a suitable point in the completion operation, the filter cake is removed to restore the formation's permeability to its original level.
Use of such conventional fluid loss additives may result in long periods of clean-up after their use. Fluid circulation, which in some cases may not be achieved, may provide a high driving force, which allows diffusion to take place to help dissolve the concentrated build up of materials. Graded salt particulates can be removed by circulating unsaturated salt brine to dissolve the particles. In the case of a gravel pack operation, if this occurs before gravel packing, the circulating fluid often causes sloughing of the formation into the wellbore and yet further loss of fluids to the formation.
In addition, under HTHP conditions, polymeric materials used to viscosify wellbore fluids and provide a measure of fluid loss control may degrade, causing changes in the rheology of the fluid and may place additional strain on wellbore equipment. Exposure to HTHP conditions can have a detrimental effect on viscosifying agents, resulting in a loss in viscosity of the fluid at high temperatures. Specialized additives for HTHP conditions often contain polymeric materials that have exceptional resistance to extreme conditions, but can involve specialized cleanup fluids to remove. For example, many cellulose and cellulose derivatives used as viscosifiers and fluid loss control agents degrade at temperatures around 200° F. (93.3° C.) and higher. Hydroxyethyl cellulose (HEC), on the other hand, is considered sufficiently stable to be used in an environment of no more than about 225° F. (107.2° C.). Likewise, because of the high temperature, high shear, high pressures, and low pH to which well fluids are exposed, xanthan gum is considered stable to be used in an environment of no more than about 290 (143.3° C.) to 300° F. (148.8° C.). However, the thermal stability of polymers such as xanthan gum may also contribute to decreased well productivity. As a result, expensive and often corrosive breaker fluids have been designed to disrupt filter cakes and residues left by these polymers, but beyond costs, the breakers may also result in incomplete removal and may be hazardous or ineffective under HTHP conditions.